Future server products require higher bandwidth interconnects on their platforms such as, for example, PCI Express Gen4 (16 Gbps), 100 GbE (4×25 Gbps), OIF-CEI-25G-LR, etc. These high speed interconnects use very short PCB trace length with FR4 Printed Circuit Board (PCB) material. However, there may be mismatches between the maximum PCB trace length achievable for a particular platform and the required PCB trace length for that platform. For example, Table 1 below shows the platform requirement for PCI Express Gen4 and 25 GbE.
TABLE 1RequiredPCB traceInterconnectslength [″]CommentPCI Express Gen42025GbE40Backplane configuration
With improved FR4 PCB material it is difficult to meet PCI Express Gen4 (PCIe4) and 25 GbE trace length requirements on high volume server platform products.
In order to reduce the gap between the desired trace length and those that are feasible, the channel loss needs to be reduced significantly and/or the repeater needs to be inserted in between two components on the channel (e.g., host and end agents). To reduce channel loss, a lower loss PCB material may be used as the substrate between the two components. This provides a great deal of improvement. Low loss PCB material is very close to meet PCIe4 trace length requirement but still does not meet the 20″ requirement. Also, low loss PCB material still cannot meet the 25 GbE trace length requirement. Very low loss PCB material can meet the PCIe4 trace length requirement with good margin. Also, very low loss PCB material can meet the 25 GbE trace length requirement of 40″ with very smooth surface treatment on copper and very wide signal traces. However, it is expensive to use low loss and very low loss PCB material and adds significant bill of materials (BOM) cost to the system. Furthermore, very low loss PCB cost is intolerable for high volume server platform products. Moreover, there are situations where even the use of a low loss PCB and/or very low loss PCB cannot meet the platform requirements. For example, PCI Express Gen4 cannot meet the 20″ requirement with high volume, low loss PCB material. Similarly, 25 GbE also cannot meet the platform requirements with very low loss PCB material where non-smooth copper surface and/or non-wide signal traces are used.
The use of a repeater can solve the issue of not meeting the signal trace length requirement for the platforms. However, the inclusion of a repeater adds cost, increases power requirements and requires more PCB area.